Electronically servo-assisted bicycle gearshifts are described in U.S. Pat. Nos. 5,480,356; 5,470,277; 5,865,454; and 6,047,230; in European patent application EP 1 103 456; and in German patent application DE 39 38 454 A1.
In particular, EP 1 103 456 describes a type of gearshift wherein the position transducers are of the absolute type, capable of providing an electrical signal indicating the absolute position of the derailleurs, this type of transducer takes into account the actual position of the derailleurs, therefore operation of the device is not detrimentally affected by displacements of the gearshift mechanism which occur when the device is switched off, due for example, to vibrations caused by the travel of the bicycle.
For correct operation of the gearshift in normal ride operating mode (i.e. wherein the gearshift is commanded manually by the rider or automatically or semi-automatically by the electronic control unit), the rear and front actuators must preliminarily be aligned in a start position, used as a reference (together with information on the position of the various sprockets and/or on the distance or pitch between adjacent sprockets) to displace the chain between adjacent sprockets to carry out the gear-shiftings. The start or reference position is usually the one in which the chain is at the sprocket with the smallest diameter.
In the prior art mechanically commanded gearshifts, the alignment in the start position is carried out with manual adjustment devices which provide for correcting the position of a steel cable which is used to actuate the displacements during a gear-shifting.
In electronically servo-assisted gearshifts, the electronic control unit, to carry out the displacement of the chain between two adjacent sprockets, drives the actuator referring to logic positions (logic values) representative of the physical positions of the various sprockets.
In these types of gearshifts, the setting of the start or reference position is usually carried out in the factory, causing the derailleur, in absence of a control signal of the actuator, to be at the sprocket with the smallest diameter.
In the event of collisions or replacement of the rear wheel, it may occur that the new rear wheel is of a slightly different size from the replaced rear wheel, in particular as far as the hub and the sprockets or pinions of the rear gearshift group are concerned.
Due to the displacement or the slightly different size, the chain and the engaged sprocket may not be perfectly aligned, with the consequences being production of noise and increased risk of incorrect operation of the gearshift itself. The front gearshift group is also subject to misalignments, especially in the case in which the replacement of the rear wheel takes place during a bicycle race; such drawbacks are important to overcome. The invention described herein seeks to overcome these and other shortcomings in the prior art.